1. Field of the Invention
The present invention relates to an improved means for affixing an article in a self-centering vacuum fixture, for use in machining the article on a lathe or the like. In particular, the present invention relates to a machinable datum for use in a vacuum fixture, in which the vacuum fixture comprises:
a cylindrical stationary mandrel portion having a central cavity therethrough, and comprising:
a first end mountable to a machine tool, such as a lathe;
a second end terminating in a contact datum intended for contact with a workpiece; and,
vacuum means capable of introducing a vacuum into a space between said contact datum and said workpiece;
a cylindrical plunger portion residing within said central cavity of said cylindrical stationary mandrel portion, said cylindrical plunger portion comprising:
a first end approximately coextensive with said first end of said stationary mandrel portion, comprising a biasing spring;
a plunger comprising a cylindrical plunger residing within said central cavity of said cylindrical stationary mandrel portion, and biased away from said first end of said stationary mandrel portion by said biasing spring;
and, a second end terminating in a contact datum intended for contact with a workpiece; and wherein at least one of said contact datums are comprised of an easily machinable material which can be shaped to facilitate contact with a workpiece.
2. Description of Related Art
Advanced high performance shaped-charge warheads are used in a variety of military systems as the mechanism to defeat a wide variety of military targets. The basic shaped-charge warhead consists of a shaped charge liner, usually conical in shape, inserted into an explosive charge. Upon detonation, the liner collapses on its axis causing a high-speed jet of liner material to be propelled from the front of the warhead. The outside contour of the explosive charge may take on a variety of shapes and configurations to either enhance warhead performance, or to fit within the confines of a particular munitions system. Various other components may be incorporated within the explosive charge to shape or alter the passing detonation wave in order to enhance performance.
For high performance shaped-charge warheads, the explosive charge is fabricated to near net shape and then machined on a CNC lathe to final configuration. The ability to solidly hold the warhead in the lathe, and the accuracy with which the warhead is spun on the centerline of the shaped-charge liner dictate how precisely the overall dimensions of the warhead can be machined. The precision to which the warhead can be machined has a significant effect on penetration performance since any asymmetry in the warhead will cause the shaped-charge liner to collapse off axis and skew the resultant jet formation. With the continued call for lighter, smaller and more lethal warheads, the need for more exacting machining and assembly procedures has become even more critical. It is extremely difficult to meet the exacting tolerances required using current methods to fixture the warhead in the lathe.
In general, warheads can be fixtured in a lathe by mechanical or vacuum methods. Warheads that are machined using mechanical fixturing devices, such as a three-jaw chuck, are relatively inaccurate, since they rely on the outside contour of the warhead for alignment. There are also several vacuum fixturing techniques for machining warheads that are more accurate than mechanical means. However, each of these vacuum fixturing techniques has certain drawbacks.
One method of vacuum fixturing involves holding the warhead on a vacuum plate, using the liner face for alignment. In doing so, however, accuracy is dependent on strict perpendicularity between the axis of the shaped-charge liner and the face of the shaped-charge liner. It is further dependent upon the degree to which the warhead can be manually centered on the vacuum plate.
A second method involves machining a vacuum mandrel that fits inside the shaped-charge liner. Although the mandrel spins true because it was machined on the same lathe, it is difficult to match the exact contour of any individual liner, let alone a group of liners. Any misalignment will have a tendency to allow the warhead to move on the fixture, thus reducing accuracy.
A third method involves a spring-loaded vacuum fixture that uses two datum plates to align the warhead on its own axis. Although substantially more accurate than the other methods described, this method still has two drawbacks which limit its effectiveness. The first is that heretofore the datum plates have each been fashioned to accommodate a particular type of shaped-charge liner, which has created resulted in a high cost for their manufacture. Secondly, each time a change in the manufacturing operation is desired, such as a change from the manufacture of one kind of shaped-charge liner to another, the fixture must be changed. This requires the installation of a new fixture or a change in the datum associated with the existing fixture, and the fixture must be manually realigned and trued before manufacturing can commence.
Objects of the Present Invention
It is an object of the present invention to provide datums for a self-centering vacuum fixture that can easily be fashioned to accommodate a variety of objects, such as different shaped-charge liners.
It is another object of the present invention to provide datums for a self-centering vacuum fixture that can be employed without the realignment and truing problems that have been associated with vacuum fixtures.
The other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiment thereof.
According to the preferred embodiment of the present invention, there is provided a machinable datum for use in a vacuum fixture, which vacuum fixture comprises:
a cylindrical stationary mandrel portion having a central cavity therethrough, and comprising:
a first end mountable to a machine tool, such as a lathe;
a second end terminating in a contact datum intended for contact with a workpiece; and,
vacuum means capable of introducing a vacuum into a space between said contact datum and said workpiece;
a cylindrical plunger portion residing within said central cavity of said cylindrical stationary mandrel portion, said cylindrical plunger portion comprising:
a first end approximately coextensive with said first end of said stationary mandrel portion, comprising a biasing spring;
a plunger comprising a cylindrical plunger residing within said central cavity of said cylindrical stationary mandrel portion, and biased away from said first end of said stationary mandrel portion by said biasing spring; and,
a second end terminating in a contact datum intended for contact with a workpiece;
and wherein at least one of said contact datums are comprised of an easily machinable material which can be shaped to facilitate contact with a workpiece.